Carbon dioxide (CO2) is a well-known greenhouse gas and attempts to reduce the emissions of this gas into the atmosphere are desirable. CO2 is commonly formed when hydrocarbonaceous assets are converted into hydrocarbonaceous products, e.g., hydrogen or power. As an example, a gas-to-liquids (GTL) process converts roughly two-thirds of the starting gas (methane or natural gas) into hydrocarbonaceous liquid products with the other one-third being emitted as CO2. The current high costs associated with capturing and sequestering this CO2 using conventional amine scrubbing technology coupled with sequestration of high pressure CO2 are such that doing so is generally not economically-viable. Accordingly, it is desirable to reduce both CO2 emissions and the costs associated with their sequestration. Toward this end, it has been proposed that CO2 be captured when electrical power is generated from hydrocarbonaceous assets, as for example in the integrated-gasification-combined-cycle (IGCC) process. See, e.g., U.S. Pat. No. 5,666,800.
Methods to capture and mitigate the entrance of CO2 into the atmosphere have primarily focused on amine scrubbing from flue gas or super-atmospheric gas streams coupled with compression of the CO2 prior to sequestration underground. This presents problems. First, the costs to compress the CO2 can be significant. Second, there are questions as to whether or not the CO2 sequestered in underground reservoirs will, in fact, remain there.
one approach to reduce greenhouse gas emissions is to substitute a crop-based biofuel for a petroleum-derived fuel. In preparing the crop-based biofuel, CO2 is consumed during the plant growth cycle. For example, there is interest in ethanol production from corn, and biodiesel from various grains. The problems with this crop-based approach include: (1) diversion of scarce farmland that is engaged in growing food for manufacture of transportation fuels; (2) use of scarce fresh water for the production of biofuels (in the United States, the decline of the Ogallala aquifer due to agricultural use could restrict future agriculture); and (3) the energy used to create the finished biofuel (i.e., product) reduces the net energy production, wherein associated energy utilization steps include fertilization, planting, harvesting, drying, milling, fermenting, extracting, distilling, transesterification and the like (some studies have indicated that there is no net energy production from ethanol).
An alternative to crop-based biofuels is to use a photobiofuels process which converts the CO2 into liquid hydrocarbonaceous products by use of photosynthetically-responsive microbes (“microbes”). A photobiofuels process, in the context of this invention, is a biological process employing microorganisms such as algae (e.g., microalgae) and/or diatoms (e.g., phytoplankton) to convert carbon dioxide into liquid hydrocarbonaceous products such as triglycerides, alcohols, acids, mono-esters and other oxygenated compounds. In doing this, the photobiofuels process uses sunlight as an energy source to produce lipids (triglycerides) and carbohydrates (e.g., sugars and starches). The photobiofuels process can also produce oxygen as a by-product. Photobiofuels processes can be characterized as open or closed, as described below.
An open photobiofuels process is one in which an aqueous liquid containing the algae and/or diatoms is in direct contact with the atmosphere. This is typically done using ponds. An advantage of an open photobiofuels process is its relatively low cost. Disadvantages include an inability to collect the produced O2, an inability to prevent contamination of the aqueous liquid with native microbes, and difficulty in controlling the temperature. An excellent example of a photobiofuels process can be found in “A Look Back at the U.S. Department of Energy's Aquatic Species Program: Biodiesel from Algae” by Sheehan et al. (referred to herein as the “Sheehan report” or simply “Sheehan”). This document was prepared by the National Renewable Energy Laboratory as NREL/TP-580-24190.
A closed photobiofuels process is one in which the aqueous liquid containing the algae and/or diatoms is not in direct contact with the atmosphere, but is instead protected by a transparent structure which permits light to enter. Advantages of the closed photobiofuels process include the ability to collect produced O2, protection of the liquid from introduction of native microbes, and improved ability to control the temperature. The primary disadvantage of this process is cost (see Sheehan, Technical Review pages 245-246). Examples of closed photobiofuels processes can be found in the following U.S. Patent Application Publications by Berzin: US20050260553, US20050239182, and US20050064577—collectively referred to herein as the “Berzin patents” or simply “Berzin”.
Looking at the open photobiofuels process described in Sheehan, several attractive features have been discovered: (1) CO2 from coal-fired power plants can be converted into a photobiofuel (Executive Summary, page i); (2) the CO2 from the coal-plants was a 13% concentration and bubbled into ponds containing the microbes (Program summary page 4); (3) concentrated, high pressure CO2 sources in power plants, synthetic fuels plants, and IGCC plants were found to be the most economical sources (Technical Review, page 216); (4) >90% of injected CO2 is consumed (Executive Summary, page ii); (5) the process does not use fresh water, but rather uses more abundant saline water that cannot be used in conventional agriculture (Program Summary, page 10); (6) the yield per acre of biofuel is thirty times that which can be obtained for crop-based biofuels (Program Summary, page 3); (7) the resulting algae can contain 60 wt % triglycerides—a photobiofuel pre-cursor (Program Summary, page 6); (8) oxygen is made as a by-product, but this can act to inhibit microbe growth (Technical Review, page 181); (9) the reagents needed to support growth of the microbe (minerals and nitrogen) can be recycled (Technical Review, page 145); (10) methane or ethanol can be produced from fermentation of biomass that does not yield triglycerides (Program Summary, page 6); and (11) the triglycerides can also be used as valuable specialty chemicals (Technical Review, page 1).
Despite its attractiveness, several problems have been identified or associated with the above-described open photobiofuels process: (1) the microbes may only grow well under rather narrow conditions of salinity, pH, and temperature (Technical Review, page 16); (2) low nighttime temperatures can limit productivity (Executive Summary, page ii); (3) yearly temperature cycles (growing season) can also limit productivity (Technical Review, page 213); (4) zooplankton can act as grazers and eat the microbes which generate the photobiofuel (Technical Review page 152), and such grazers can be, a particular problem at night (Technical Review, page 180); (5) carbon dioxide is only consumed during the day when sunlight is available; (6) the cost of the produced biodiesel was estimated (in 1995 dollars) as being between $1.40 to $4.40/gallon; and even with carbon credits, this was judged to be twice the cost of petroleum diesel, and therefore not competitive (Program Summary, page 19; and Executive Summary, page ii); (7) an analysis in the report concluded, that it will be difficult to find many locations where all the resources required for microalgae cultivation (e.g., flatland, brackish or waste waters, and low cost CO2 supplies) are all available in juxtaposition (Technical Review, page 259).
One approach to improve upon the economics of such above-described processes is to use a closed photobiofuels process. This affords a better chance to control the temperature, salinity, pH and microbial species. Examples of this are shown the Berzin patents. However, am economic analysis of closed systems performed by Sheehan concluded that the costs of these systems were prohibitive (Program Summary, page 5)
While the price of petroleum diesel has increased markedly since the above described 1995 study and might now make this viable, efforts to improve the economics of such photobiofuels process are still desirable. The approach taken herein to improve such economics is to enhance the integration of the processes for conversion of hydrocarbonaceous assets and the photobiofuels process. Improved economics are achieved either by lower cost operations, improved productivity, improved value of the product, and combinations thereof.